Downlink Frequency Synchronization Algorithm For OFDM Systems

Orthogonal frequency division multiplexing (OFDM) realizes high-speed communication using many parallel sub-carriers. In OFDM systems, a transmission channel is partitioned into a set of orthogonal sub-channels, each with approximately flat transfer function and additive white Gaussian noise (AWGN). OFDM is a bandwidth efficient signaling scheme for digital communications and can effectively counteract multipath fading. At present it has been regarded as a core technique for the fourth generation (4G) mobile communication system. However, all sub-carriers are completely independent, which is achieved only if carrier and sampling frequency at the transmitter and receiver are perfectly synchronized. This paper takes frequency offset estimation for OFDM systems as the research direction. It has the following innovative points:Firstly, a new OFDM system model is proposed in this paper when the effect of sampling frequency offset on system performance is discussed. The model considers the effects of consecutive multiple OFDM symbols as well as the cyclic prefix. The final conclusion is that phase rotation and inter-carrier interference caused by sampling frequency offset will get worse as OFDM symbol number, not just sub-carrier number, increases.Secondly, the Maximum Likelihood (ML) estimator based the cyclic prefix (CP) is improved by exchanging more computational data for high estimation precision of frequency offset estimation.Thirdly, an estimation method making use of two training symbols in time domain is improved from three aspects including symbol timing, estimation of fractional carrier frequency offset and integer carrier frequency offset. Lastly, a robust frequency synchronization scheme is proposed for estimation of joint carrier frequency offset (CFO) and sampling frequency offset (SFO) in OFDM systems. Specifically, carrier and sampling frequency are generated in a common crystal oscillator. The estimation algorithm is based on phase locked loop (PLL) using only one single training symbol. In addition, the PLL measures the timing error which can be used to measure the frequency error resulted from the different channel frequencies between the transmitter and receiver so as to improve the precision of frequency synchronization.